which the plant absorbs carbon dioxide and gives out oxygen in the carbon assimilation process (photosynthesis). They are in fact the openings through which the exchange of all gases takes place, although to some extent the whole surface of the leaf and even the stem functions in this capacity. When the external covering or “cuticle” of the leaf is thick, however, the process is largely confined to the stomata. Usually these occur more thickly on the under-surface of the leaf, as being better protected from the drying influence of the sun. Only in water-plants with floating leaves are they confined to the top surface. Although the number of stomata found on the leaves of different plants varies greatly, there do not appear to be any fewer on those belonging to halophytes or xerophytes than on the leaves of normal plants. A moderately large leaf with an average density of stomata may possess several millions of such openings.
Although there is much which is obscure about the transpiration process, it has two important effects. Firstly, it maintains a constant flow of water from root to leaf through the wood of the plant, bringing with it also small quantities of dissolved salts which are essential for the plant’s growth. Secondly, it tends to reduce the temperature of the leaf when it is exposed to the heat of the sun. It is a well-known fact that when a liquid is changed into vapour, energy (latent heat) has to be expended. This heat is derived from the air immediately in contact with the surface of the leaf and in this way the leaf itself is cooled. In hot climates and in dry habitats this result may be important. The chief danger with xerophytes, and to a lesser extent with halophytes, is that the loss of water by transpiration may be so rapid that it cannot be replaced from the scanty supply of water available at their roots. Many plants belonging to both these classes are therefore equipped with devices to check excessive transpiration, and some of these will now be described.
TRANSPIRATION-CHECKS
OR DEVICES FOR REDUCING TRANSPIRATION
The development of a thick cuticle or outer skin on the leaves is the simplest and most frequently adopted method for the reduction of transpiration. The leathery feel of the leaves produced by many seaside plants is a characteristic which can hardly be overlooked, though the development of thick cuticles is by no means confined to coastal plants. In some cases this thickening is supplemented by the secretion of wax on the leaf surface, as in the case of the sea-holly (Eryngium maritimum) (Pl. 1). These protective layers have the effect of confining the evaporation of water entirely to the stomata, for in their absence a considerable amount of water is lost through the rest of the surface. Fig. 3 shows diagrammatically a transverse section round a stoma of a leaf with a thin cuticle (a) and a similar section from a leaf with a thick cuticle (b). It will often be noticed that the thickness of the cuticle varies considerably amongst individuals of the same species, according to the habitat in which they are growing. The leaves of the scarlet pimpernel (Anagallis arvensis) (Pl. 2b), for instance, become thick and leathery when it is growing on bare sand amongst dunes, although under normal conditions in garden soil they are soft and slender.
FIG. 3.—Types of stomata: a. Transverse section of leaf with a thin cuticle: b. Transverse section of leaf with a thick cuticle, showing a sunken stoma; c. Surface view of a stoma.
In many plants the stomata are protected by being placed in grooves or hollows sunk well below the surface of the leaf (Fig. 3(b)). In the dune grasses, marram-grass (Ammophila arenaria) and sea lyme-grass (Elymus arenarius), the stomata are mostly confined to the bottom and sides of the deep grooves in their leaves. This protection is much improved by the tendency of the leaves to roll up into a narrow tube in dry weather, which has the effect of maintaining a layer of air, largely saturated with water-vapour, between the stomata and the outside air, and thus reducing evaporation. The manner in which air is enclosed when the leaf rolls up is clearly shown in Fig. 4, and the corrugated inner (i.e. upper) surface is due to the deep grooves along which the stomata are scattered. The outer (i.e. under) surface is furnished with a thick cuticle and is devoid of stomata. This habit of rolling the leaf under dry conditions is shared by many plants, and is a good example of the way they can adjust themselves to variations in their water-supply. When water is plentiful, the blade opens out and becomes flat, thus exposing a greater surface for transpiration. The fresh appearance of marram-grass on open sand-dunes after abundant rain is quite distinct from its parched look after a long period of dry weather, and on closer inspection will be found to be due to the unfolding of its leaves.
FIG. 4.—Transverse section of marram-grass leaf when rolled (from Fritsch & Salisbury, 1946).
Another common way in which the stomata are protected is by the growth of hairs on the surface of the leaf. These are often associated with sunken stomata and are very effective in maintaining a damp atmosphere round the opening, since moisture tends to condense on them. The stiff hairs protecting the furrows on the upper surface of the marram-grass leaf will be noticed in Fig. 4. Many seaside plants have hairy leaves, and some are covered with a thick down. The yellow horned poppy (Glaucium flavum) (Pl. IX), the sea stock (Matthiola sinuata) and the buck’s horn plantain (Plantago coronopus) (Pl. XXXVI) are good examples of coastal plants with hairy leaves, while the leaves of sea-wormwood (Artemisia maritima) (Pl. XXXI) and the tree-mallow (Lavatera arborea) are markedly downy. The characteristic silvery foliage of the sea-buckthorn (Hippophae rhamnoides) (Pl. XX) and sea-purslane (Halimione (Obione) portulacoides) (Pl. 16) is also due to scale-like (peltate) hairs covering the surface of the leaves. These hairs are usually dead when the leaf is mature, and contain only air. Apart from aiding the retention of moist air near the surface, they reflect much of the sun’s heat. Some leaves possess simple unbranched hairs, but those on many others are branched and occur in very different forms. Some typical covering hairs from the leaves of coastal plants are shown in Fig. 5. Like the thickness of the cuticle, the degree of hairiness shown by individuals of the same species often varies with availability of the water-supply in the habitat. Thus the sand-dune form of silverweed (Potentilla anserina) commonly shows a thick felting of silvery hairs on the upper surface of its leaves, as well as on the lower.
FIG. 5.—Typical covering-hairs on various leaves: a. Plantago coronopus; b. Cynodon dactylon; c. Erophila verna; d. Matthiola sinuata; e. Hippophae rhamnoides.
Still another way in which relatively damp air is maintained over the surface of the leaves is by the plant adopting a dense mat habit, so that the transpiring surfaces of the leaves are kept in close contact with each other. Alpine plants often mass their foliage in this way, but amongst coastal plants thrift (Armeria maritima) provides one of the best examples, since its habit varies considerably with the place in which it is growing. Thus the close rosette form is typical when it is growing on rocky cliffs and other dry habitats, or when it is heavily grazed, whilst with a better water-supply it assumes a much more open habit (Fig. 6). Many sand-dune plants spend most of the year in the form of a rosette, only sending up a vertical stem during the flowering season. In this way, only the upper surface of the leaf is exposed
